Evaluation of thermal damage and mechanical properties of P91 steel in service using nonlinear ultrasonic waves

Pressure vessels and high-temperature pipelines inevitably experience thermal damage and mechanical property degradation under high-temperature and high-pressure service conditions. Using P91 steel specimens subjected to long-term high-temperature (HT) service as the research subject, this study employs nonlinear ultrasonic testing (NUT) technology to propose a rapid and accurate uniaxial mechanical property evaluation method. The influence of thermal damage on the ultrasonic nonlinear parameter and its response mechanism in P91 steel during prolonged high-temperature service was investigated by using a NUT system on P91 steel specimens with varying degrees of thermal damage. Analysis of the variation of the nonlinear parameter revealed that the ultrasonic nonlinear parameter first increases, then decreases, and then increases again with the duration of high-temperature service. This study introduces a cumulative ultrasonic nonlinear coefficient and establishes a mapping relationship model between the high-temperature mechanical property degradation of P91 steel and the cumulative ultrasonic nonlinear coefficient, thereby enabling the rapid assessment of the mechanical properties of P91 steel in long-term high-temperature service using NUT. The validity of this mapping relationship was confirmed through conventional tensile tests. The results demonstrate that the error between the yield strength and tensile strength calculated based on nonlinear ultrasonic technology and the tensile test results is within 10 %, meeting engineering application requirements.